Transmitting apparatus and method for receiving a digital signal in a digital telecommunication system

ABSTRACT

A receiver that receives a reference symbol comprising a sequence of a plurality of synchronization repetition patterns, wherein each repetition pattern contains a predetermined number of samples. The reference symbol is part of a digital signal modulated by using OFDM modulation. An end synchronization repetition pattern in the reference symbol is phase-shifted by 180°, and the phase-shifted synchronization repetition pattern is positioned after the sequence of the number of synchronization repetition patterns. The receiver detects a timing of a correlation peak at the end of said reference symbol by performing a cross-correlation of the synchronization repetition patterns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. Ser. No. 14/037,829, filed Sep. 26, 2013which is a continuation application of U.S. Ser. No. 13/618,613, filedon Sep. 14, 2012 which is a continuation of Ser. No. 11/429,210, filedon May 8, 2006 (now U.S. Pat. No. 8,861,622), which is a continuation ofU.S. Ser. No. 11/210,527, filed on Aug. 23, 2005, which is acontinuation-in-part of U.S. Ser. No. 09/510,652, filed Feb. 22, 2000(now U.S. Pat. No. 7,154,975), which claims the benefit of priorityunder 35 U.S.C. §119 from European Patent Application No. 99103546.0,filed Feb. 24, 1999 the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a transmitting apparatus and atransmitting method, for transmitting a digital signal in a digitaltelecommunication system. The present invention is hereby particularlydirected to the generation and transmission of a reference symbol whichis used on a receiver side to achieve a time and/or frequencysynchronisation.

Digital telecommunication systems generally need a synchronisation of atransmitting side and a receiving side. The transmitting side and thereceiving side can e.g. be base stations and mobile stations of atelecommunication system, whereby the synchronisation of the timing andthe frequency of transmitted signals is usually performed in the mobilestation. To achieve a synchronisation, it is known to transmit a specialtraining sequence or a reference symbol, also called synchronisationsymbol. Such a reference symbol is usually embedded in the transmissiondata structure and regularly sent so that a synchronisation can beperformed regularly.

In FIG. 1, a general structure of a receiving apparatus is shown inorder to explain the synchronisation mechanism on which the presentinvention is based. The receiving apparatus can e.g. be a mobile stationof a wireless digital telecommunication system. Although the presentinvention essentially relates to the transmitting part of atelecommunication terminal, it is to be understood, that thetransmitting part or transmitting apparatus of the present invention canalso be a or part of a receiving and transmitting terminal.

The receiving apparatus 1 shown in FIG. 1 comprises an antenna 2 forreceiving signals from a transmitting side, e.g. a base station of awireless digital telecommunication system. The received signals 2 aresupplied to a HF means (High Frequency means) 3, which downconverts thereceived high frequency signals into the base band. The downconvertedsignals are supplied to a IQ-demodulation means 4, where they aredemodulated and supplied to a synchronising means 5.

The synchronising means performs time and frequency synchronisationusing a received training sequence or reference symbol, as stated above.Using the synchronization information of the synchronising means 5, thereceived user data signals are further processed in the receivingapparatus 1, e.g. decoded by a decoding means 6 and so on, to be madeavailable in visible or audible form for a user. Usually thesynchronisation in the synchronising means 5 is performed in the timedomain.

Generally speaking, the synchronising means 5 performs a time domaincorrelation between the reference symbol (or parts of the referencesymbol) and a delayed version of the received reference symbol (or partsof the reference symbol) to identify the reference symbol (or parts ofthe reference symbol) and thus to determine the timing for thesynchronisation. Thereby, a correlation peak is calculated, which shouldcorrespond as accurate as possible to the time point of the last sampleof the reference symbol.

In order to achieve a well detectable correlation peak, the referencesymbol usually consists of a plurality of synchronisation patterns,which are repeated several times within one reference symbol period. Thesynchronisation patterns usually have the same shape or form and arethus called repetition patterns throughout the present application. Areference symbol therefore contains several repetition patterns, wherebyeach repetition pattern consists of a plurality of samples. Eachrepetition pattern has the same number of samples. Between the referencesymbol and the adjacent user data symbols, guard intervals can beinserted to avoid intersymbol interference in a multipath environment ofthe telecommunication system.

The time domain correlation of the received reference symbol in thereceiving apparatus 1 can be achieved e.g. on the basis of an autocorrelation mechanism or a cross correlation mechanism. An autocorrelation mechanism thereby does not require any knowledge about thereference symbol on the receiver side, whereby a cross correlationmechanism requires exact knowledge about the reference symbol to bereceived on the receiver side.

A known cross correlation means 7 is shown in FIG. 2. The crosscorrelation means 7 cross correlates incoming signals y(i), e.g. comingfrom the IQ demodulation means 4, within a cross correlation window of alength 16. The cross correlation window length 16 means that theincoming digital signal y(i) is cross correlated sample by sample on thebasis of a length of 16 samples. The cross correlation window length of16 samples can thereby correspond to the length of a repetition patternof the reference symbol. In FIG. 3, a reference symbol comprising 9repetition patterns is shown, whereby one repetition pattern cancomprise 16 samples. The receiving apparatus 1 knows exactly thestructure of the reference symbol to be received. A complex conjugatedversion of an expected repetition pattern is stored in the synchronisingmeans 5 and cross correlated to the received signals.

The cross correlation means 7 of FIG. 2, which has a cross correlationwindow length of 16, comprises 15 delay means 8 arranged serially. Thefirst delay means delays the incoming complex signal y(i) by one sample,which corresponds to multiplication with a factor z⁻¹. The second delaymeans delays the output of the first delay means again by 1 sample andso on. Further, the cross correlation means 7 comprises 16multiplication means 9 and a sum means 10. The delay means 8, themultiplication means 9 and the sum means 10 are arranged so that anincoming signal having a length of 16 samples is cross correlated with acomplex conjugated version of the samples of a repetition pattern. Thecomplex conjugated samples of the expected repetition pattern are e.g.stored in the synchronising means of the receiver and read outrespectively to the multiplication means 9. E.g. a first received sampley(0) is multiplied with a complex conjugated version of the first sampleof the expected repetition pattern, i.e. y*(0)=s₀*. The next receivedsample y(1) is multiplied with y*(1)=s₁* and so forth. The sum means 10adds up all the results from the multiplication means 9, so that anoutput signal r(i) is obtained. The output signal r(i) of the sum means10 is supplied to an absolute value calculating means 11 whichcalculates the absolute value of r(i) to detect a cross correlationpeak. The cross correlation means 7 and the absolute value calculatingmeans 11 shown in FIG. 2 can be comprised in the synchronising means 5of the receiving apparatus 1 shown in FIG. 1.

In FIG. 3, the cross correlation peak detection performed by the crosscorrelation means 7 and the absolute value calculating means 11 shown inFIG. 2 is explained. FIG. 3 shows three different phases of a crosscorrelation calculation of an incoming signal. In phase 1, thecorrelation window 13 of the cross correlation means 7 is located onreceived user data, which means that only user data are crosscorrelated. The user data are indicated by “??? . . . ”. Thus, no crosscorrelation peak is detected. In phase 2, the correlation window 13 isexactly matching with the eighth repetition pattern S7 of the referencesymbol 12, so that a corresponding cross correlation peak is detected.In phase 3, the cross correlation window 13 is again cross correlatinguser data “??? . . . ”, so that no cross correlation peak is detected.

The reference symbol 12 shown in FIG. 3 comprises 9 repetition patternsS0, S1, . . . , S8, which have identical shapes. Each of the repetitionpatterns comprises e.g. 16 samples, which corresponds to the crosscorrelation window length 16 of the cross correlation means 7 in FIG. 2.Of course, the number of repetition patterns in the reference symbol 12and the number of samples in each repetition pattern can be changed andadopted to the respective application.

As stated above, the cross correlation mechanism requires exactknowledge on the reference symbol to be received on the receiving side.This means, that the receiving apparatus needs to know exactly thestructure and number of repetition patterns to be able to recognise thelast cross correlation peak, which serves for a time and frequencysynchronisation. On the other hand, if one of the cross correlationpeaks is not properly detected, the synchronisation fails. In mobilecommunication environments, in which multipath fading degrades thecorrelation peak detection performance, the synchronisation performancein a known receiving apparatus of the telecommunication system is thussignificantly lowered.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to provide a transmittingapparatus and transmitting method for transmitting a digital signal in adigital telecommunication system which generate a reference symbol whichallows for an improved time and/or frequency synchronisation performanceand accuracy on the receiving side.

According to a first aspect of the invention, a transmitting apparatusfor transmitting a digital signal in a digital telecommunication systemcomprises means for preparing a reference symbol comprising a sequenceof a plurality of synchronisation repetition patterns, whereby eachrepetition pattern contains a predetermined number of samples, means fortransmitting said reference symbol as a part of said digital signal byusing OFDM (Orthogonal Frequency Division Multiplexing) modulation to areceiver side apparatus, wherein an end synchronisation repetitionpattern in said reference symbol is phase-shifted by 180° and saidreference symbol comprises a number of said synchronisation repetitionpatterns and said phase-shifted synchronisation repetition pattern ispositioned after the sequence of said number of synchronisationrepetition patterns so that the receiver side apparatus can exactlydetect a timing of a correlation peak at the end of said referencesymbol by performing a cross correlation of said synchronisationrepetition patterns. Advantageously, the transmitting apparatus of thepresent invention further comprises adjusting means for increasing thetransmission power when transmitting the reference symbol.

According to a further aspect of the present invention, a method fortransmitting a digital signal in a digital telecommunication systemcomprises the steps of preparing a reference symbol comprising asequence of a plurality of synchronisation repetition patterns, whereineach repetition pattern contains a predetermined number of samples,transmitting said reference symbols as a part of said digital signal byusing OFDM (Orthogonal Frequency Division Multiplexing) modulation to areceiver side apparatus, wherein an end synchronisation repetitionpattern in said reference symbol is phase-shifted by 180° and saidreference symbol comprises a number of said synchronisation repetitionpatterns and said phase-shifted synchronisation repetition pattern ispositioned after the sequence of said number of synchronisationrepetition patterns so that the receiver side apparatus can exactlydetect a timing of a correlation peak at the end of said referencesymbol by performing a cross correlation of said synchronisationrepetition patterns.

Advantageously, the method according to the present invention furthercomprises the step of increasing the transmission power whentransmitting the reference symbol.

According to a further aspect of the present invention, a transmittingapparatus for transmitting a digital signal in a digitaltelecommunication system comprises means for preparing a referencesymbol comprising a sequence of a plurality of synchronisationrepetition patterns, wherein each repetition pattern contains apredetermined number of samples, means for transmitting said referencesymbol as part of said digital signal by using OFDM (OrthogonalFrequency Division Multiplexing) modulation to a receiver sideapparatus, wherein an end synchronisation repetition pattern in saidreference symbol is phase-shifted by 180° and said reference symbolcomprises a number of said synchronisation repetition patterns and saidphase-shifted synchronisation repetition pattern is positioned after thesequence of said number of synchronisation repetition patterns so thatthe receiver side apparatus can perform a synchronisation process inaccordance with said synchronisation repetition patterns and exactlydetect the timing of said end of the reference symbol by performing across-correlation of said synchronisation repetition patterns.

Advantageously, the transmitting apparatus of the present inventionfurther comprises adjusting means for increasing the transmission powerwhen transmitting the reference symbol.

According to a further aspect of the present invention, a method fortransmitting a digital signal in a digital telecommunication systemcomprises the steps of preparing a reference symbol comprising asequence of a plurality of synchronisation repetition patterns, whereineach repetition pattern contains a predetermined number of samples,transmitting said reference symbol as a part of said digital signal byusing OFDM (Orthogonal Frequency Division Multiplexing) modulation to areceiver side, wherein an end synchronisation repetition pattern in saidreference symbol is phase-shifted by 180°) and said reference symbolcomprises a number of said synchronisation repetition patterns and saidphase-shifted synchronisation repetition pattern is positioned after thesequence of said number of synchronisation repetition patterns so thatthe receiver side can perform a synchronisation process in accordancewith said synchronisation repetition patterns and exactly detect atiming of said end of said reference symbol by performing across-correlation of said synchronisation repetition pattern.

Advantageously, the method according to the present invention furthercomprises the step of increasing the transmission power whentransmitting the reference symbol.

According to a further aspect of the present invention, a transmitterdevice for transmitting OFDM (Orthogonal Frequency DivisionMultiplexing) signals to a receiver in an OFDM system comprises meansfor preparing a reference symbol comprising a sequence of a plurality ofsynchronisation repetition patterns, wherein each repetition patterncontains a predetermined number of samples, means for transmitting saidreference symbol as part of said digital signal by using OFDM modulationto a receiver side apparatus in said OFDM system, means for preparing areference symbol comprising a plurality of successive repetitionpatterns, whereby said reference symbol is transmitted from saidtransmitter device by using multicarriers of said OFDM system and a lastrepetition pattern of said successive repetition patterns isphase-shifted in relation to the other repetition patterns, and whereineach of said successive repetition patterns generated by said generatingmeans is composed of the same number of samples, so that saidsynchronisation repetition patterns transmitted to said receiver sidedevice are cross-correlated in said receiver side device in order toperform time and frequency synchronisation in said receiver side device.

Advantageously, in the transmitter device according to the presentinvention, the last repetition pattern of said successive repetitionpattern is phase-shifted by 180° in rotation to the other repetitionpatterns. Further advantageously, the transmitter device according tothe present invention further comprises adjusting means for increasingthe transmission power when transmitting the reference symbol.

According to a further aspect of the present invention, a method fortransmitting OFDM (Orthogonal Frequency Division Multiplexing) signalsto a receiver side in an OFDM system comprises the steps of preparing areference symbol comprising a sequence of a plurality of synchronisationrepetition patterns, wherein each repetition pattern contains apredetermined number of samples, transmitting said reference symbol aspart of said digital signal by using OFDM modulation to a receiver sidein said OFDM system, preparing a reference symbol comprising a pluralityof successive repetition patterns, whereby said reference symbol istransmitted from a transmitter side by using multicarriers of said OFDMsystem and a last repetition pattern of said successive repetitionpattern is phase-shifted in relation to the other repetition patterns,and wherein each of said generated successive repetition patterns iscomposed of the same number of samples, so that said synchronisationrepetition patterns transmitted to said receiver side arecross-correlated on said receiver side in order to perform time andfrequency synchronisation on said receiver side.

Advantageously, in the method according to the present invention, thelast repetition pattern of said successive repetition patterns isphase-shifted by 180° in relation to the other repetition patterns.Further advantageously, the method according to the present inventionfurther comprises the step of increasing the transmission power whentransmitting the reference symbol.

According to a further aspect of the present invention, a transmitterdevice for transmitting OFDM (Orthogonal Frequency DivisionMultiplexing) signals in an OFDM telecommunication system comprisesmeans for generating said OFDM signals having a reference symbolcomprising a plurality of successive repetition patterns, wherein a lastrepetition pattern of said plurality of successive repetition patternsis phase-shifted in relation to the other repetition patterns, and meansfor transmitting said generated OFDM signals including said referencesymbol and transmitting data to a receiver side device, wherein each ofsaid plurality of successive repetition patterns generated by saidgenerating means is composed of the same number of samples,respectively, so that said repetition patterns transmitted to saidreceiver side device are cross-correlated in said receiver side devicein order to perform time and frequency synchronisation in said receiverside.

Advantageously, in the transmitter device according to the presentinvention, the last repetition pattern of said plurality of successiverepetition patterns is phase-shifted by 180° in relation to the otherrepetition patterns. Further advantageously, the transmitter deviceaccording to the present invention further comprises adjusting means forincreasing the transmission power when transmitting the referencesymbol.

According to a further aspect of the present invention, a method fortransmitting OFDM (Orthogonal Frequency Division Multiplexing) signalsin an OFDM telecommunication system comprises the steps of generatingsaid OFDM signals having a reference symbol comprising a plurality ofsuccessive repetition patterns, wherein a last repetition pattern ofsaid plurality of successive repetition patterns is phase-shifted inrelation to the other repetition patterns, and transmitting saidgenerated OFDM signals including said reference symbol and transmittingdata to a receiver side, wherein each of said generated successiverepetition patterns is composed of the same number of samples so thatsaid repetition patterns transmitted to said receiver side arecross-correlated on said receiver side in order to perform time andfrequency synchronisation on said receiver side.

Advantageously, in the method according to the present invention, thelast repetition pattern of said successive repetition patterns isphase-shifted by 180° in relation to the other repetition patterns.Further advantageously, the method according to the present inventionfurther comprises the step of increasing the transmission power whentransmitting the reference symbol.

It is to be noted that the use of a sequence of a plurality ofsynchronisation repetition patterns in the reference symbolsignificantly enhances the time and frequency synchronisationperformance and accuracy as compared to the provision of only a fewrepetition patterns. Further, by phase-shifting the last synchronisationrepetition pattern in the reference symbol by 180° in relation to allother synchronisation repetition patterns in the reference symbol, avery accurate and reliable phase detection on the receiver side and thusan accurate time and/or frequency synchronisation is possible.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is explained in detail in the followingdescription by means of preferred embodiments relating to the encloseddrawings, in which

FIG. 1 shows the general structure of a receiving apparatus of a digitaltelecommunication system,

FIG. 2 shows a known cross correlation means and absolute valuecalculation means for detecting a cross correlation peak,

FIG. 3 shows the cross correlation peak detection performed by the crosscorrelation structure of FIG. 2,

FIG. 4 shows the structure of a reference symbol used forsynchronisation according to the present invention,

FIG. 5 shows the cross correlation peak detection using the referencesymbol shown in FIG. 4,

FIG. 6 shows a transmitter structure according to the present invention,

FIG. 7 shows a cross correlation means and a detection means fordetecting cross correlation peaks and respective phase information onthe basis of a reference symbol as shown in FIG. 4,

FIG. 8 shows a cross correlation means and another detection means fordetecting a single cross correlation peak on the basis of a referencesymbol as shown in FIG. 4,

FIG. 9 shows a simulation result for the absolute value of an outputsignal of the structure shown in FIG. 8,

FIG. 10 shows a further embodiment of the detection means of FIG. 7,

FIG. 11 shows a simulation result of the cross correlation means and thedetection means of FIG. 10,

FIG. 12 shows a further embodiment of a cross correlation meansaccording to the present invention together with an absolute valuecalculation means,

FIG. 13 shows a simulation result of the cross correlation means and theabsolute value calculation means shown in FIG. 12 for detecting a crosscorrelation peak,

FIG. 14 shows a further embodiment of a synchronising structureaccording to the present invention comprising a cross correlation meansaccording to the present invention and a peak threshold detection meansand a gap detection means, and

FIG. 15 shows an alternative structure to the embodiment shown in FIG.14.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 shows the structure of a reference symbol 14 as example for areference symbol structure to be used according to the presentinvention. The reference symbol 14 of FIG. 4 comprises 9 synchronisationrepetition patterns S0, S1, . . . S8. Each repetition pattern has alength of 16 samples s₀, s₁, . . . s₁₅. Thereby, the last repetitionpattern S8 is phase-shifted by 180 degrees in relation to the otherrepetition patterns, which means a multiplication by (−1). Thus, thelast repetition pattern S8 comprises 15 samples −s₀, −s₁, . . . −s₁₅.All synchronisation repetition patterns of the reference symbol 14 havethe same shape, i.e. identical content, whereby the last repetitionpattern S8 is phase-inverted by 180 degrees in relation to the otherrepetition patterns of the reference symbol. All other (preceding)synchronisation repetition patterns have the same phase. It is to benoted, that the reference symbol 14 can have more or less than 9repetition patterns and that each repetition pattern can have more orless than 16 samples.

In FIG. 5, the reference symbol 14 is shown to be embedded in a userdata sequence. The reference symbol 14 can hereby be inserted in anywanted or advantageous location within a sequence of data symbols.Between the reference symbol and the data symbols before and after thereference symbols, a so-called guard interval can be inserted in orderto avoid inter-symbol interference (ISI) in a multipath environment. Inthe time domain the reference symbol 14 has a length N and eachsynchronisation repetition pattern has a length of N_(sp), so that thereference symbol 14 consists of (N/N_(sp)) copies of the synchronisationrepetition pattern. A very efficient way of generating reference symbolsof the desired structure, e.g. in an OFDM (Orthogonal Frequency DivisionMultiplexing) transmission system, is the application of an IFFT(Inverse Fast Fourier Transformation) exploiting the properties of theDFT (Discrete Fourier Transformation) algorithm. Consequently, in orderto generate a reference symbol of length T_(s) with (N/N_(sp))synchronisation repetition patterns of length THN_(sp)/N only every(N/N_(sp))-th DFT coefficient (every N/N_(sp)-th subcarrier in thefrequency domain) has to be modulated. At the beginning and/or at theend of a reference symbol 14, a guard interval may be inserted in orderto avoid inter-symbol interference (ISI). Hereby, the guard interval canbe formed by a cyclic extension of each symbol by copying the last fewsynchronisation repetition patterns.

The user data are indicated by “??? . . . ”. FIG. 5 shows threedifferent phases of cross correlating a received signal having areference symbol 14, in which the last repetition pattern S8 isphase-inverted by 180°. Relating to the receiving apparatus 1 shown inFIG. 1, the data sequence of the three phases shown in FIG. 5 are forexample supplied from the IQ demodulation means 4 to the synchronisingmeans 5, whereby the synchronising means 5 is e.g. constructed as shownin FIG. 7. In phase 1, the cross correlation window 15 cross correlatesonly user data, so that no cross correlation peak is detected. In phase2, the 8th repetition pattern S7 of the reference symbol 14 is matchedby the correlation window 15, so that a cross correlation peak isdetected. The relative phase of the cross correlation peak of the 8threpetition pattern S7 is also detected to be “+”. Since the 9threpetition pattern S8 is phase-inverted by 180° in relation to the 8threpetition pattern S7, the cross correlation peak detected for the 9threpetition pattern S8 has the relative phase “−” in relation to thephase of the 8th repetition pattern S7. The repetition patterns S0, S1 .. . S6 preceding the two last repetition patterns S7 and S8 have arelative phase “+”.

In phase 3 of FIG. 5, only user data are cross correlated in the crosscorrelation window 15, so that no cross correlation peak is detected. Ascan be seen in FIG. 5, by using a reference symbol structure like theone shown in FIG. 4, in which one of the repetition patterns isphase-inverted in relation to at least one of the other repetitionpatterns in the reference symbol, a relative phase information can beobtained additional to the cross correlation peak information. Thisphase information provides additional information on the position of thelast correlation peak in the reference symbol and thus a more accurateand reliable synchronisation information.

FIG. 6 shows a transmitting apparatus or transmitting device 60according to the present invention. To be precise, FIG. 6 showsimportant elements of a transmitting apparatus 60 according to thepresent invention which are necessary to explain and to understand thepresent invention. Data to be transmitted are supplied to a channelencoder 61. The output of the channel encoder 61 is supplied to areference symbol insertion circuit 62. In the reference symbol insertioncircuit 62, the reference symbols from a memory 64, where they arestored, are multiplexed by a multiplexer 63 with the data to betransmitted. The output from the reference symbol insertion circuit 62is supplied to an OFDM (Orthogonal Frequency Division Multiplexing)burst mode controller 15. The output from the OFDM burst mode controller65 is given to an inverse FFT circuit 66. The output from the inverseFFT circuit 66 is supplied to a power adjustment circuit 67. In thepower adjustment circuit 67, the transmitting power is increased when areference symbol is transmitted. The output from the power adjustmentcircuit 67 is supplied to a synchronisation repetition pattern rotation(inverting) circuit 68. The synchronisation repetition pattern rotationcircuit 68 contains a circuit 69 for extracting the last synchronisationrepetition pattern of a reference symbol, a phase shifter 70 and acombining circuit 71 combining the phase shifted last synchronisationrepetition pattern of a reference symbol with the other synchronisationrepetition patterns in the same reference symbol. The output of thesynchronisation repetition pattern rotation circuit 68 is supplied to acircuit 72 which inserts a cyclic extension into the reference symbol.Then the data stream containing the data to be transmitted as well asthe reference symbols is modulated by a modulator 73 on a radiofrequency (RF). After filtering the data to be transmitted in a filter74 the filter data are given to an RF-front-end stage 75. The referencesymbols are inserted into the data in the frequency domain to avoid thegenerally large implementation effort when inserting the referencesymbols of the data in the time domain.

The average power of the reference symbol upon transmission is lowerthan the average power of other OFDM-symbols due to the lower number ofmodulated subcarriers. Therefore, the adjustment circuit 67 is providedin order to increase the transmitting power to match the averagetransmission power of the OFDM-data symbols. This can be achieved by amultiplication of each sample of the reference symbol with a poweradjustment factor which calculates to F_(power)=√{square root over(N/N_(sp))}. After the power adjustment the last synchronisationrepetition pattern is rotated by 180°, which is realised through amultiplication by −1 in the synchronisation repetition pattern rotationcircuit 68. After the complex signal is converted into a real signal bythe IQ-modulator 73, it is passed to the transmission RF-front-end stage75 in order to be transmitted through an antenna over a wireless link toa receiving device, which is e.g. disclosed in the following figures.

In FIG. 7, a cross correlation means 16 and a detection means 19 areshown, which can be implemented in a first embodiment of a synchronisingmeans 5 of a receiving apparatus 1 of the present invention, the generalstructure of which is shown in FIG. 1. The structure of the crosscorrelation means 16 is identical to the structure of the crosscorrelation means 7 shown in FIG. 2, so that a detailed explanation isomitted. The cross correlation means 16 comprises 15 delay means 17 and16 multiplication means 18 as well as a sum means for adding the outputsof the multiplication means 18. The cross correlation window length ofthe cross correlation means 16 corresponds to the length of onerepetition pattern, which is e.g. 16 samples. A received data stream of16 samples is cross correlated with complex conjugated samples of anexpected repetition pattern stored in the receiving apparatus 1. Theoutput signal r(i) of the sum means, i.e. the output signal of the φcross correlation means 16 is supplied to a detection means 19 fordetecting the magnitude and the phase of the signal r(i) and thereforethe exact position of the cross correlation peak of the last repetitionpattern S8 of the reference symbol 14 can be detected (cf. FIG. 5).

FIG. 8 shows another arrangement of the detection means. The crosscorrelation means 16 of FIG. 8 corresponds to the cross correlationmeans 16 of FIG. 7. In the example shown in FIG. 8, the detection meanscomprises a delay means 20 for delaying the output signal r(i) of thecross correlation means 16 by one repetition pattern length, which ise.g. 16 samples. The detection means 19 further comprises a subtractionmeans 21 for subtracting the output signal s(i) of the delay means 20from the output signal r(i) of the cross correlation means 16. Theoutput signal z(i)=r(i)−s(i) of the subtraction means 21 is supplied toan absolute value calculation means 22, which calculates the absolutevalue of z(i). It is to be noted, that y(i), r(i), s(i), z(i) arecomplex values so that the magnitude and the phase information iscontained in z(i). If it is assumed, that r(i) is in the part of thereference symbol, in which the phase of the repetition patterns is notphase-shifted, for example in the part S0, . . . S7 of the referencesymbol 14 shown in FIG. 4, then s(i)r(i−16)=r(i)•e^(iφ)Ψz_(J)(i)=r(i)−s(i)=r(i)(1−e^(iφ)).

If it is assumed, that r(i) matches with the phase-inverted repetitionpattern S8 of the reference symbol 14, then s(i)=r(i−16)=−r(i)•e^(iφΨz)₂(i)=r(i)−s(i)=r(i)(1+e).

It can be seen that the absolute value of z(i) is enhanced if r(i)matches with the phase-shifted repetition pattern S8. The phase value φhas nothing to do with the phase shift between the repetition pattern S7and S8, but results from a possible frequency offset between thetransmitter side and the receiver side. Considering the detection rangeof the phase change introduced by the reference symbol structureaccording to the present invention under the influence of a frequencyoffset between the transmitter and the receiver, the following result isobtained: z₁(i)/z₂(i)=−j·cot(φ/2). Thus, for a non-ambiguous detectionthe absolute value of φ has to be smaller than π, whereby the phasevalue c is the product between the frequency offset and the durationT_(P) of one repetition pattern, φ=2πf_(offset)T_(P).

In FIG. 9, a simulation result for the absolute value of z(i) as theoutput signal of the structure shown in FIG. 8 is shown. For thereference symbol 14 comprising 9 repetition patterns, whereby eachrepetition pattern consists of 16 samples, and whereby the phase of thelast repetition pattern S8 is inverted in relation to the phase of theother repetition patterns, the cross correlation peak is expected to beat the last sample, i.e. the time point corresponding to the lastsample, of the last repetition pattern S8. As can be seen in FIG. 9, thecross correlation peak is located at sample 144, which is the correctvalue. Thus, the cross correlation means 16 and the detection means 19shown in FIG. 9 and in FIG. 8 enable a correct and efficient detectionof the cross correlation peak.

In FIG. 10, the cross correlation means 16 and another embodiment of thedetection means of FIG. 8 are shown. Thereby, the structure shown inFIG. 10 corresponds to the structure shown in FIG. 8, whereby the outputof the absolute value calculating means 22 is supplied to an averagingmeans 23 for smoothening the absolute value of z(i) output from themeans 22. The structure shown in FIG. 10 is particularly advantageous insevere noise and fading environments. The averaging means 23advantageously is a moving average filter having a filter lengthcorresponding to the length of one repetition pattern, which is forexample 16 samples as shown in FIG. 4. The cross correlation structuresshown in FIGS. 8 and 10 can e.g. be implemented in the synchronisingmeans 5 of the receiving apparatus 1 shown in FIG. 1.

FIG. 11 shows a simulation result for the averaged absolute value ofz(i) as the output signal of the structure shown in FIG. 10. Thedetection of the last repetition pattern having an inverted phase asshown in FIG. 4 can be seen in the transition between sample 128 andsample 144.

In FIG. 12, a second embodiment of a cross correlation means 24 isshown, which CM be implemented in a synchronising means 5 of a receivingapparatus 1 of the present invention, a general structure of which ise.g. shown in FIG. 1.

The cross correlation means 24 essentially has the same structure as thecross correlation means 16 shown in FIG. 7 and the cross correlationmeans 7 shown in FIG. 2. The main difference is, that the crosscorrelation means 24 shown in FIG. 12 has a cross correlation windowlength of two repetition patterns, which in the shown examplecorresponds to 32 samples, when the structure of the reference symbolshown in FIG. 4 is assumed. Thereby, the cross correlation means 24comprises 31 delay means 25, which are arranged serially andrespectively cause a delay of one sample. Further, the cross correlationmeans 24 comprises 32 multiplication means, which multiply therespective (delayed) samples of the received signal y(i) with storedpositive and negative complex conjugated values of the samples of theexpected repetition pattern. Thereby, e.g. the first sample entering thecross correlation means 24 is multiplied with the first complexconjugated sample s₀* of the expected repetition pattern. The same istrue for the rest of the samples entering the cross correlation means24, which are respectively multiplied with the rest of the stored(positive) complex conjugated samples s₁* to s₁₅*. The second 16 samplesentering the cross correlation means 24 are respectively multiplied willthe stored negative complex conjugated samples −s₀* to −s₁₅* of theexpected repetition pattern. Hereby, e.g. the first sample entering themeans 24 is multiplied with the negative value of the complex conjugatedfirst sample of the expected repetition pattern −s₀*. The same is truefor the rest of the second 16 samples entering the means 24 which arerespectively multiplied with the negative values of the complexconjugated values, namely −s₁* to −s₁₅*. It is to be noted, that thevalues s₀, s₁, . . . , s₁₅ of the repetition patterns S0, S1, . . . ,S8, of the reference symbol 14 shown in FIG. 4 are respectively thesame. In other words, all the repetition patterns S0, S1, . . . , S8 ofthe reference symbol 14 of FIG. 4 have the same shape, except that thelast repetition pattern S8 has an inverted phase.

The outputs of the multiplication means 26 of the cross correlationmeans 24 are added up in a sum means 27, which generate an output signalz(i). The output signal z(i) of the sum means 27 is supplied to anabsolute value calculation means 28, which calculates the absolute valueof z(i). The output signal of the absolute value calculation means 28therefore provides information on the magnitude as well as on the phaseof the data signals, which are cross correlated by the cross correlationmeans 24.

A simulation result for the output of the absolute value calculationmeans 28 of the structure shown in FIG. 12 is shown in FIG. 13. In thiscase, a reference symbol similar to the reference symbol 14 shown inFIG. 4 had been used, but only with 6 repetition patterns, whereby eachrepetition pattern consists of 16 samples. The phase of the lastrepetition pattern is shifted by 180° in relation to the other precedingrepetition patterns. Thus, the position of the last sample of the lastrepetition pattern is expected to be at sample position number 96, whichis clearly visible in the simulation result shown in FIG. 13. FIG. 13shows clearly, that the output signal has a maximum exactly when acorrect overlapping between the two repetition patterns processed in thecross correlation means 24 is achieved.

FIG. 14 shows an extended structure for increasing the reliability andaccuracy of the output signal of the absolute value calculation means 22of the structure shown in FIG. 8, the averaging means 23 of thestructure shown in FIG. 10 or the absolute value calculation means 28 ofthe structure shown in FIG. 12. In the improved structure shown in FIG.13, the respective output signal of the cross correlation means 24 orthe detection means 19, which is the absolute value of z(i), is suppliedto a peak threshold detection means 29 and a gap detection means 30. Thepeak threshold detection means 29 detects if the absolute value of z(i)exceeds a predetermined cross correlation peak threshold. The gapdetection means 30 detects if the absolute value of z(i) has been belowa predetermined gap threshold before said detected cross correlationpeak. In FIG. 13 it can be seen, that the absolute value of z(i) is zeroor close to zero as long as the data signals entering the crosscorrelation means are in the part of the reference symbol, where thephase of the repetition patterns is not inverted in relation to eachother. Hereby, a presynchronisation can be achieved, since the detectedcorrelation peak is only confirmed when the gap in front of thecorrelation peak is detected.

In other words, the gap in front of the correlation peak can be used toidentify the (range for the possible position of the cross correlationpeak. Only when the peak threshold detection means 29 detects that theabsolute value of z(i) exceeds the predetermined cross correlationthreshold and the gap detection means detects that the absolute value ofz(i) has been below a predetermined gap threshold before the detectivecross correlation peak, the cross correlation peak is confirmed. In thiscase, the peak threshold detection means 29 and the gap detection means30 send respectively a positive information to a determination means 33,which can for example be an AND gate, which outputs the position of thedetected cross correlation peak only in case of a positive signal fromboth of the means 29 and 30. In front of the gap detection means 30, anaveraging means 31 and/or a delay means 32 can be located. The averagingmeans 31 can for example be a moving average filter to smoothen theabsolute value of z(i). The filter length preferably corresponds to thelength of one repetition pattern of the reference symbol. The delaymeans 32 preferably provides a delay corresponding to the length of onerepetition pattern of the reference symbol. The averaging means 31 aswell as the delay means 32 can be provided or not depending on theapplication.

FIG. 15 shows an alternative structure to FIG. 14. In FIG. 15, theabsolute value of z(i) is supplied to a peak threshold detection means29 identical to the peak threshold detection means 29 of FIG. 14. Thegap detection means 34 shown in FIG. 15 detects if the absolute value ofz(i) has been below a predetermined gap threshold before the detectedcross correlation peak and additionally detects if it has been below thepredetermined gap threshold during a predetermined gap time. In thecontrary to the gap detection means 30 of FIG. 14, which only checks onetime point before the detected cross correlation peak, the gap detectionmeans 34 of FIG. 15 checks a time period before the detected crosscorrelation peak. Identically to FIG. 14, a determination means 33,which can for example be an AND gate, determines if the output signalsfrom the peak threshold detection means 29 and the gap detection means34 are both positive and confirms the detected correlation peak to bethe required correlation peak for that case. Both structures shown inFIGS. 14 and 15 provide an increased detection accuracy and reduce thefalse alarm possibility by combined detection of a presynchronisationand a correlation peak detection. The presynchronisation, i.e. thedetection of the gap in front of a detected cross correlation peakenables to detect the range of possible synchronisation peak positions,what can be used to reduce the number of computations needed for thesucceeding synchronisation.

It has to be noted, that although the cross correlation andsynchronisation structures shown in FIGS. 7, 8, 10, 12, 14 and 15 can beimplemented in the synchronising means 5 of the receiving apparatus 1shown in FIG. 1, these inventive structures can be implemented or usedin any other receiving apparatus as long as the scope of the presentinvention as defined in the enclosed claims is met.

The invention claimed is:
 1. A method comprising: receiving a referencesymbol comprising at least nine synchronization repetition patterns,wherein each synchronization repetition pattern contains a predeterminednumber of samples, the reference symbol is part of a digital signalmodulated by using OFDM modulation, and an end synchronizationrepetition pattern in the reference symbol is phase-shifted by 180° inrelation to the other synchronization repetition patterns included inthe reference symbol; and detecting a timing of a correlation peak atthe end of the reference symbol by performing a cross-correlation of theplurality of synchronization repetition patterns.
 2. A methodcomprising: receiving a reference symbol comprising at least ninesynchronization repetition patterns, wherein each synchronizationrepetition pattern contains a predetermined number of samples, thereference symbol is part of a digital signal modulated by using OFDMmodulation, and an end synchronization repetition pattern in thereference symbol is phase-shifted by 180° in relation to the othersynchronization repetition patterns included in the reference symbol;performing a synchronization process by detecting a timing of an end ofthe reference symbol by performing a cross-correlation of the pluralityof synchronization repetition patterns.